1. Field of Invention
This invention relates to attachment systems for reflecting surfaces used to concentrate solar energy.
2. Description of Prior Art
Reflecting surfaces have been used to concentrate solar energy for many decades. One of the earliest applications is said to have been the use of polished shields by hundreds of warriors to concentrate the sun on enemy sails, thereby setting fire to the ships.
A heliostat is, by definition, a reflecting surface that continuously reflects the sun to a designated or fixed point. In the technology of solar thermal energy use, heliostats are formed of single or multiple reflecting panels that are usually planar or moderately focussed, and are affixed to a moveable supporting structure. Fields of many heliostats may be used to achieve high concentrations of solar energy on a central receiver. Heliostats do not track the sun directly, but split the angle between the sun and the receiver.
In addition to the concentrating reflectors such as heliostats, parabolic troughs, and parabolic dishes, that move with the sun there are several types of solar concentrators that utilize fixed reflectors. Fixed reflectors may benefit from the attachment methods described herein.
Mirrored glass is one of the best reflectors for solar energy concentrators. Many years and many millions of dollars have been spent trying to improve on mirrored glass. However, second-surface (back-surfaced) mirrored glass remains one of the most efficient, long-lived and cost effective reflectors.
One of the problems with mirrored glass has always been attaching the glass to the support structure. Glass mirrors have usually been attached to the support structure by gluing supports to the back side of a glass sandwich, with the mirrored surface in between the two glass panels; by using a glass substrate with a front reflector surface; or by gluing supports with a large contact surface area to the backside of a second-surface mirror.
One of the principal approaches to concentrator design by these inventors has been the use of off-the-shelf components to construct the machines. This approach led to the development of a support attachment using off-the-shelf elevator bolts that are directly attached to the glass surface by means of an off-the-shelf long-lived silicone building sealant. For example, with a 30 inch.times.30 inch glass panel, an array of four one-quarter inch diameter elevator bolts with a one inch diameter flat head has worked very successfully. This scheme leads to a low cost, high strength fixture method with simple means of adjusting the reflecting panel on its support structure. Heliostat assemblies constructed this way have withstood extreme wind and weather for long periods of time. A potential problem with the elevator bolt approach is that with a second-surface mirror (A second-surface mirror is required to obtain the durability benefits of glass reflectors) the supports are most efficiently attached to the mirrored side of the glass.
A typical mirrored surface on glass comprises a layer of silver deposited on the glass, a protective layer of copper deposited over the silver, and a protective paint applied to the copper. Thus, the bonding of the support to the glass is only as strong as the weakest of the glass-silver, copper-silver, or paint-copper bonds. Since the foregoing bonds may be relatively weak and can vary considerably in strength, a support attachment with a large contacting surface is required to successfully attach sufficiently strong supports to the multi-layered back surface.
Years of successful testing have demonstrated that the attachment of small supports directly to the glass is a technically and economically practical way to support the glass. Recently, direct joints to glass were achieved by simply removing the paint, copper, and silver from small spots on the mirrors and attaching the supports directly to glass. A disadvantage of this method is the loss of a small amount of reflecting surface. For example, in a 30 inch by 30 inch square mirror panel, approximately four square inches of reflecting surface might be removed. This can be compensated for by adding about one sixteenth of an inch to the 30 inch length of each side. The bolt heads have been set in a sealant thickness of approximately 0.020-inch to provide resilience between the glass and bolt head.
In using a thin glass panel with this type of support, deflections in the glass may be introduced by moments on the supports. Deflections can distort the reflected image in undesirable ways. Undesirable deflections in the glass can be overcome by the use of slightly oversized holes in the support structure for all but one of the supports on an individual reflector panel, and supporting the glass panel against a specified surface jig while the supports are adjusted to provide the desired focus. The resiliency in the adhesive that is attaching the support to the reflector panel helps to prevent glass distortion from small deflections of the support.
Several methods of attaching reflecting surfaces to support structures are the subject of U.S. Patents. These include:
U.S. Pat. No. 4,435,043, Mertens, et al. This approach provides for a backing sheet attached to the reflective surface. The supports are then attached to this backing sheet. The only potential advantange of our invention over this method is simplicity.
U.S. Pat. No. 4,501,469, Merges, et al. This approach provides for focussing of the panel by means of pins attached to the reflector. The reflecting surface is not removed, hence a larger and more complex support and attachment surface are expected to be required.
Focussing within individual panels is provided for in the following patents:
U.S. Pat. No. 3,906,927, Caplan. Provides for adjusting the focus through multiple components of a panel, with some adjustment within individual panels. Does not provide for attachment of supports directly to the glass.
U.S. Pat. No. 4,179,193, Gillette. Flexible membrane stretched over a hoop structure with backing membrane stretched over opposite side. Focussed by changing pressure in the enclosed volume.
U.S. Pat. No. 437,378, Anderson. Compensates for temperature changes by having front and back sheets having the same coefficient of thermal expansion, with adhesive having shear tolerance. Does not contemplate attachment directly to the glass.
U.S. Pat. No. 4,422,723, Williams, et al. Membrane using partial vacuum to focus.
U.S. Pat. No. 4,511,215, Butler. Stretched membrane, with springs, attachment to membrane with means for changing position of attachment to change the focus.
U.S. Pat. No. 4,251,135, Stone. A flexible triangular reflector sheet under tension. Does not involve removing reflector surface to attach supports.